modify /etc/network/interfaces:

auto wlan0
allow-hotplug wlan0
iface wlan0 inet dhcp
    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf
iface default inet dhcp

config in /etc/wpa_supplicant/wpa_supplicant.conf:

...
network={
        ssid="MARS"
        psk="*************"
        key_mgmt=WPA-PSK
}